1. Field of the Invention
The present invention relates to an image display device and a method of controlling the same, and more particularly, to an image display device suitable for use in a multidisplay system in which an image is displayed using a plurality of image display devices connected to each other and a method of controlling such an image display device.
2. Description of the Related Art
Image display devices for use in conjunction with computers or the like and display devices of the dot-matrix type such as liquid crystal displays or plasma displays are widely used. In those display devices, the number of pixels is fixed. Therefore, the resolution (the number of pixels) of an input image signal must be converted to an optimum resolution, depending on the number of pixels of a display device being used.
FIG. 12 is a functional block diagram of a common image display device having the capability of resolution conversion. In FIG. 12, reference numeral 100 denotes an image output unit which outputs an image signal, and 200 denotes an image display device. The image display device 200 includes an image input interface 201, a resolution converter 202, a display device driver 203, a controller 204, and a display device 205.
In this system, an image signal output from the image output unit 100 is input to the image input interface 201 of the image display device 200. If the image signal input from the image output unit 100 is an analog signal, the image input interface 201 converts the input image signal into digital form. On the other hand, in a case in which the image signal input from the image output device 100 is a digital transmission signal according to a digital signal transmission standard such as TMDS, the image input interface 201 outputs the digital signal to the resolution converter 202 via a receiver.
In a case in which the resolution of the display device 205 is, for example, XGA (1024×768 pixels) and the input image signal has a resolution of, for example, SVGA (800×600 pixels) lower than the resolution of the display device 205, the resolution converter 202 scales up the number of pixels in the horizontal and vertical directions of the input image signal by a factor of 1.28 thereby generating an image display signal having the same number of pixels as that of the display device 205.
Conversely, if the resolution of the input image signal is, for example, UXGA (1600×1200) greater than that of the display device 205, the resolution converter 202 scales down the number of pixels in the horizontal and vertical directions of the input image signal by a factor of 0.64 thereby generating an image display signal having the same number of pixels as that of the display device 205.
The controller 204 controls the scaling-up/down process described above. The display device driver 203 displays an image on the display device 205 in accordance with the image display signal generated by the resolution converter 202.
As is well known, an operating system (OS) installed on a personal computer has the capability of automatically determining an optimum format in which an image signal is output from the computer to a display connected to the personal computer. This capability is known as the plug and display capability. To achieve the plug and display capability, an image display device has a memory (such as a memory 206 shown in FIG. 12) in which attribute information associated with displaying is stored, and a computer determines, on the basis of the attribute information, an optimum format in which an image signal is supplied from the computer to the image display device, wherein the computer acquires the attribute information by performing DDC (Display Data Channel) communication with the image display device. (DDC is an extended interface standard which allows transmission of information from a display to a host computer thereby making it possible to use a video signal in an extended manner). This attribute information is called EDID information. EDID stands for Extended Display Identification Data and is a specification for transmission of information associated with a display from the display to a host computer. A format in which data is transmitted for the above purpose is defined in EDID. For example, EDID information includes information indicating the resolution of an image display device and the frequencies of horizontal and vertical scanning signals. Thus, if an image display device is formed so as to have the plug and display capability and if the resolution and other necessary information associated with the image display device is described in the EDID information, a computer (image output unit) can determine an optimum format in which an image signal is supplied from the computer to the image display device.
A multidisplay system is known in the art. In this system, a plurality of image display devices are arranged, for example, in the form of an M×N array, and a single image is displayed using those image display devices. The multidisplay system has advantages that a large screen can be easily achieved, a depth is smaller than that of a single display having a corresponding large screen size, and high brightness can be achieved. Because of the advantages described above, the multidisplay system is used in various applications, such as displays for use in exhibitions or on advertising towers, in which a large-sized display is needed. The technique of displaying an image using a multidisplay system allows for a single high-resolution display device by combining a plurality of low-resolution display devices.
An example of a multidisplay system is disclosed in Japanese Patent Laid-Open NO. 2000-148080. The multidisplay system disclosed in Japanese Patent Laid-Open NO. 2000-148080 is described below with reference to FIG. 13.
In FIG. 13, reference numeral 1001 denotes a multidisplay interface circuit serving as a display control apparatus, and reference numeral 1002 denotes a display device. In this figure, suffixes -1, -2, . . . , -n following reference numeral 1001 are used to denote similar multidisplay interface circuits, and suffixes -1, -2, . . . , -n following reference numeral 1002 are used to denote similar display devices. The multidisplay interface circuit 1001 includes an input data processor 1004, a data output unit, a control data processor 1031 and a control unit.
The data output unit includes a frame memory write circuit 1007, a frame memory read circuit 1008, a data selector 1009, frame memories 1010 and 1011, and an up-scaler 1012. The control unit includes a horizontal write start position register 1017, a horizontal write length register 1018, a vertical write start position register 1019, a vertical write length register 1020, scale-up factor registers 1021 and 1022, a horizontal read position register 1023, a vertical read position register 1024, a horizontal synchronization register 1025, a vertical synchronization register 1026, an output timing signal generator 1014, a microcomputer 1028, an ID setting circuit 1029 and a data storage memory 1030.
Image data transferred from the image output unit is written into the frame memory 1010 or 1011 in the data output unit via the input data processor 1004. In this process of writing the image data, memory areas into which the image data is written is controlled by the horizontal and vertical write start position registers 1017 and 1019 and the horizontal and vertical write length registers 1018 and 1020 in the controller. The image data is read from the frame memory 1010 or 1011 and transferred to the display device 1002 after being scaled up.
The microcomputer 1028 in the control unit extracts commands contained in control data received via a control bus, and the microcomputer 1028 transfers data stored in the data storage memory 1030 into respective registers 1017 to 1026 of the control unit. Those commands specify which parts of the image data should be displayed by the respective display devices 1002-n (n=1, 2, . . . ). In some cases, the command in the control data includes an ID number indicating which one of display interface circuits 1001 in the multidisplay system should execute the command. In this case, the microcomputer 1028 compares the ID number included in the command with an ID number in the ID setting circuit 1029, and the microcomputer 1028 executes the command if they are identical to each other.
In the present multidisplay system, as described above, while the same image data is input to all display interface circuits of the multidisplay system, the respective display interface circuits capture display data of different display areas specified by the control data and display the captured data.
At present, some personal computers have the capability of outputting a signal with as high a resolution as QXGA (2048×1536 pixels). Display devices used in conjunction with such high-performance personal computers are needed to have the capability of displaying images with corresponding high resolution.
In conventional display devices of the dot-matrix type, even if image signals applied to display devices have high resolution, images displayed on the display devices are not as high in resolution as the input image signals, because the resolution of display devices is not high enough. One technique for displaying an image having as high resolution as that of an original signal is to increase the number of pixels of a display device. However, it is expensive to develop such a high-performance display device and a high performance controller for controlling the display device.
One possible technique of solving the above problem is, as disclosed in Japanese Patent Laid-Open NO. 2000-148080, to display an image using a multidisplay system including a plurality of display devices. However, in conventional multidisplay systems, it is necessary to input a display control signal together with an image signal to respective display devices in order that a correct part of an image is displayed on each display device.